Handheld LIBS analyzer end plate purging structure

ABSTRACT

A handheld LIBS analyzer includes a laser source for generating a laser beam and a spectrometer subsystem for analyzing a plasma generated when the laser beam strikes a sample. A nose section includes an end plate with an aperture for the laser beam, a purge cavity behind the aperture fluidly connected to a source of purge gas, and a shield covering the purge cavity. A vent removes purge gas from the purge cavity when the end plate is placed on the sample.

RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 14/179,670 filed Feb. 13, 2014 and claims thebenefit of and priority thereto under 35 U.S.C. §§119, 120, 363, 365,and 37 C.F.R. §1.55 and §1.78 and is incorporated herein by thisreference, which is a continuation-in-part of U.S. patent applicationSer. No. 13/746,102 filed Jan. 21, 2013, which is incorporated herein byreference. This application is also related to U.S. patent applicationSer. No. 13/746,110 filed Jan. 21, 2013; Ser. No. 13/746,102 filed Jan.21, 2013; Ser. No. 13/746,095 filed Jan. 21, 2013; and Ser. No.13/746,108 filed Jan. 21, 2013.

FIELD OF THE INVENTION

The subject invention relates to spectroscopic instruments.

BACKGROUND OF THE INVENTION

Various spectroscopic instruments are known. X-ray based instruments,for example, can be used to determine the elemental make up of a sampleusing x-ray florescence spectroscopy. Portable XRF has become apreferred technique for elemental analysis in the field. Portable XRF isfast, non-destructive, and provides reasonably accurate results (i.e.,quantification of elemental concentrations in a wide variety ofsamples). With XRF, an x-ray tube is used to direct x-rays at a sample.Atoms in the sample absorb x-rays and re-emit x-rays that are unique tothe atomic structure of a given element. A detector measures the energyof each x-ray and counts the total number of x-rays produced at a givenenergy. From this information, the types of elements and theconcentration of each element can be deduced. Commercially availableanalyzers include the Delta manufactured by Olympus NDT and the NitonXLT-3 manufactured by Thermo Fisher Scientific.

X-rays, however, pose a safety concern. Also, portable and benchtop XRFanalyzers have not to date been used to determine lower atomic numberelements such as beryllium, sodium, carbon, boron, oxygen, nitrogen,lithium, and the like. Laser induced break down spectroscopy (LIBS)devices are known and used to detect the elemental concentration oflower atomic numbered elements with some accuracy. These devicestypically include a high powered laser that sufficiently heats a portionof the sample to produce a plasma. As the plasma cools, eventually theelectrons return to their ground states. In the process, photons areemitted at wavelengths unique to the specific elements comprising thesample. The photon detection and subsequent measurement of elementalconcentrations are similar to spark optical emission spectroscopy (OES).Examples of LIBS devices are the LIBS SCAN 25 from Applied Photonics,the LIBS25000 from Ocean Optics, and the RT 100 from Applied Spectra.See also Nos. US 2012/0044488 and WO 2013/083950 (PCT/GB2012/000892)incorporated herein by this reference.

Some elements such as carbon, phosphorous, and sulfur react with oxygenresulting in a very low level signal which can be difficult to detectand/or properly analyze.

It is known to use an inert gas such as argon to purge the sample.Typically, the flow rate is high and the area purged is large. The gasmay be used to purge a sample chamber in some prior art LIBS analysissystems. Accordingly, a large source (e.g., a tank) of argon gas isrequired and must be toted along in the field. Other analysis systemsusing an argon purge, such as a mobile spark OES analyzer, also usequite a lot of argon gas for purging.

SUMMARY OF THE INVENTION

In a LIBS device, it is desirable to use eye-safe lasers. One example ofan eye-safe laser with enough power for LIBS usage are those at 1.5micron wavelength. Other wavelengths are possible. Water absorbs heavilyat this wavelength thus preventing the laser reaching the retina of theeye. Devices with eye-safe lasers receive a regulatory rating of eitherClass 1 or Class 2 depending upon the power level of the laser. Class 1is the most desired because it is the least regulated. For handhelddevices which operate in an open beam configuration, the Class 1 orClass 2 rating is highly desired because it yields the maximum operatorsafety and is subject to the least amount of regulation.

Because of the lower pulse energies currently available from 1.5 μmlasers, it is often necessary to focus the laser into a smaller spotsize, typically 100 μm or less in order to get a high enough powerdensity to ignite a plasma. Lower power lasers than are commonly usedfor bench top WS instruments are also desirable particularly in the caseof a handheld or portable LIBS unit due to size and power restrictionsimposed to maintain portability of the instrument. The very small beamspot size on the sample creates three problems that should be solved tomake a LIBS device commercially viable. First, the laser must be focusedprecisely on the surface of the sample being analyzed for consistentanalytical results. Second, the sample must be clean from surfacecontamination including oxidation on the same distance scale of 100 μmor less. Third, some samples are non-homogeneous. Thus, on a sample,locations even a small distance away from each other my yield differentelements and/or different elemental concentrations. It is thereforedesirable to design such a LIBS device to make several measurements atdifferent regions of the sample and combine the results. The inventiondisclosed includes an eye-safe laser in one preferred embodiment.However, the invention is useful for lasers of other wavelengths and/orlarger beam spots on the sample.

In one preferred example, a spectrometer system, preferably handheld orotherwise portable, is provided and is configured to automatically,based on spectral information, properly focus the laser on the sample,clean the sample, and analyze different locations on the sample.

In a portable, battery powered device, it is not desirable to requirethe user to carry a large tank of purge gas. In one preferredembodiment, a purge subsystem allows a small argon cartridge to be used(e.g., 3-6″ long) because the purge gas is conserved. The flow rateduring testing is low and the gas flow is directed only locally to thelocation on the sample where the plasma is generated by the laser beam.Moreover, the purge gas is supplied only just before testing and turnedoff at the end of a test (or even before). In this way, the purge gas isfurther conserved.

Featured is a handheld LIBS analyzer comprising a laser source forgenerating a laser beam, a spectrometer subsystem for analyzing a plasmagenerated when the laser beam strikes a sample, and a nose section. Anend plate has an aperture for the laser beam. A purge cavity is locatedbehind the aperture and is fluidly connected to a source of purge gas. Ashield covers the purge cavity. A vent removes purge gas from the purgecavity when the end plate is placed on the sample.

In some examples the vent includes a channel in a front face of the endplate extending from the aperture. The channel may extend on oppositesides of the aperture to opposing edges of the end plate. Preferably therear of the end plate includes a rearwardly extending enclosure definingthe purge cavity. The enclosure may include at least one side openingfor receiving the purge gas. The enclosure may include a top rim forseating the shield thereon.

In some versions, the purge cavity includes a calibration standardtherein. The calibration standard may be located on a rear surface ofthe end plate. In one example, there are two calibration standardslocated on opposite sides of the laser aperture. In another example, theend plate serves as the calibration standard.

Also featured is a handheld LIBS spectrometer comprising a housing andan optics stage movably mounted to the housing, including a laserfocusing lens and a detection lens. One or more motors are configured toadvance and retract the optics stage, move the optics stage left andright, and/or move the optics stage up and down. A laser source in thehousing is oriented to direct a laser beam to the laser focusing lens. Aspectrometer subsystem in the housing is configured to receiveelectromagnetic radiation from the detection lens and to provide anoutput. A controller subsystem is responsive to the output of thespectrometer subsystem and is configured to control the laser source andthe motors. The housing preferably includes a nose section with an endplate with an aperture for the laser beam, a purge cavity behind theaperture fluidly connected to a source of purge gas, a shield coveringthe purge cavity, and a vent for removing purge gas from the purgecavity when the end plate is placed on the sample.

Preferably, the shield is made of fused silica for protecting the laserfocusing lens and detection lens of the optic stage from plasmagenerated by the laser. The handheld LIBS spectrometer may furtherinclude a gas source fluidly coupled to the purge cavity. The housingmay include a handle and the gas source may include a gas cartridgedisposed in the housing. The gas cartridge can be pivotably disposed inthe housing handle. Preferably, the gas source cartridge is fluidlycoupled to the purge cavity via a regulator, the regulator is rotatablycoupled to the housing, and the gas source cartridge is coupled to therotating regulator. The gas source may be fluidly connected to the purgecavity via a controllable valve and then the controller subsystem ispreferably configured to automatically control the valve.

The nose section may include a calibration standard for self-calibratingthe spectrometer when the controller subsystem controls the optic stageto orient the laser focusing lens to focus laser energy on thecalibration standard of the nose section. The controller subsystem canbe configured to control the optics stage motors to move the opticsstage to initiate a calibration routine. The calibration routine mayinclude computer instructions which control one or more motors to movethe optics stage to a predetermined set of coordinates, to power thelaser to produce a laser beam, to process the output of the spectrometersubsystem, and to calibrate the spectrometer. The computer instructionswhich calibrate the spectrometer include instructions which determinewavelength and/or intensity calibration constants based on the output ofthe spectrometer subsystem. The handheld LIBS spectrometer controllersubsystem may further be configured to control the optics stage motorsto move the optics stage to initiate an auto-focus routine, andauto-clean routine, a moving spot cycle, and/or a purge cycle.

The subject invention, however, in other embodiments, need not achieveall these objectives and the claims hereof should not be limited tostructures or methods capable of achieving these objectives.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features, and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a block diagram showing an example of a spectrometer system inaccordance with the invention;

FIG. 2 is a block diagram showing another example of a spectrometersystem in accordance with the invention;

FIGS. 3A and 3B are block diagrams showing still another example of aspectrometer system in accordance with the invention;

FIGS. 4A-4C are schematic views showing sample spectral intensity dataas determined by a detector subsystem in accordance with FIGS. 1-3 atthree different focusing lens positions for a technique used todetermine the optimal focusing lens position in accordance with examplesof the invention;

FIG. 5 is a graph showing the integration of spectral intensity over allwavelengths for ten different focusing lens positions;

FIG. 6A is a graph showing intensity for carbon in a steel sample duringsequential laser pulses in accordance with a cleaning method associatedwith embodiments of the invention;

FIG. 6B is a graph showing intensity for iron during sequential laserpulses in accordance with the cleaning method associated with FIG. 6A;

FIG. 7A is a view of a sample to be cleaned by the LIBS laser of FIGS.1-3 prior to performing an analysis;

FIG. 7B is a view of a portion of the sample of FIG. 7A after cleaning;

FIG. 8 is a flow chart depicting the primary steps associated with amethod in accordance with the invention and/or the programming and/orconfiguration of the controller depicted in FIGS. 1-3;

FIG. 9 is a flow chart depicting the primary steps associated with thefocusing cycle depicted in FIG. 8;

FIG. 10 is a flow chart depicting the primary steps associated with thecleaning cycle of FIG. 8;

FIG. 11 is a flow chart depicting the primary steps associated with themoving spot cycle shown in FIG. 8;

FIG. 12 is a schematic three dimensional view of a hand held batterypowered LIBS spectrometer device in accordance with an example of theinvention featuring a gas purge subsystem;

FIG. 13 is a schematic view showing a portion of the device of FIG. 12;

FIG. 14 is a block diagram showing the primary components associatedwith an example of a gas purge subsystem;

FIG. 15 is a timing diagram showing a number of laser pulses; and

FIG. 16 is a graph showing the spectrometer signal strength for a numberof purge conditions;

FIG. 17 is a schematic side view of an example of a handheld LIBSspectrometer system in accordance with the invention;

FIG. 18 is a schematic three dimensional front view of the movableoptics head of the system of FIG. 17;

FIGS. 19A and 19B show how the laser beam is moved on a sample togenerate a plasma at numerous locations detected by the detection lensof the optics head;

FIG. 20 is a schematic front view showing components of the opticalstage for moving the optics head of FIG. 18;

FIG. 21 is a schematic bottom view again showing components of theoptics stage;

FIG. 22 is a schematic front view showing additional details concerningthe optics stage;

FIG. 23 is a schematic left hand side of the optics stage and opticalhead;

FIG. 24 is a schematic bottom view showing components of the opticsstage and the optics head;

FIG. 25 is a schematic view showing the optics head coupled to fiberoptic bundles;

FIG. 26 is a schematic view showing the spectrometer subsystem enclosurearray;

FIG. 27 is a schematic top view showing a spectrometer enclosure;

FIG. 28A is a schematic view showing a gas cartridge pivotably mountedin the handheld LIBS spectrometer handle portion;

FIG. 28B is a schematic view showing the gas cartridge being pivoted outof the LIBS spectrometer handle portion for replacement;

FIG. 29 is a schematic view showing the bottom of the handheld LIBSspectrometer handle with a pair of batteries removable therefrom;

FIG. 30 is a block diagram showing the primary components associatedwith one preferred handheld LIBS spectrometer system in accordance withthe invention;

FIG. 31 is a flow chart depicting the primary steps of a calibrationroutine in accordance with a method of the invention and in accordancewith computer instructions operating on the controller subsystem show inFIG. 30;

FIG. 32 is a flow chart depicting a method in accordance with thesubject invention and also depicting the primary steps associated withcomputer instructions executed by the controller subsystem shown in FIG.30;

FIG. 33 is a block diagram showing an example of the primary electroniccomponents associated with the controller subsystem of FIG. 30;

FIG. 34 is a block diagram showing an example of the primary electroniccomponents associated with the processor module of FIG. 33;

FIG. 35 is a block diagram showing an example of the primary electroniccomponents of the laser driver board of FIG. 33;

FIG. 36 is a cross sectional view showing a handheld LIBS analyzer endplate disposed on a sample to be analyzed;

FIG. 37 is a schematic front view of a handheld LIBS analyzer with animproved end plate;

FIG. 38 is a schematic three dimensional rear view of the end plateshown in FIG. 37;

FIG. 39 is a schematic view showing a sample and the analysis locationsthereon in accordance with an example of the invention;

FIG. 40 is a schematic view of the rear of the end plate now mounted toa portion of the nose section of the handheld LIBS analyzer; and

FIG. 41 is a schematic front view showing another example of a LIBSanalyzer end plate with purge gas venting channels formed therein.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, thisinvention is capable of other embodiments and of being practiced orbeing carried out in various ways. Thus, it is to be understood that theinvention is not limited in its application to the details ofconstruction and the arrangements of components set forth the followingdescription or illustrated in the drawings. If only one embodiment isdescribed herein, the claims hereof are not to be limited to thatembodiment. Moreover, the claims hereof are not to be read restrictivelyunless there is clear and convincing evidence manifesting a certainexclusion, restriction, or disclaimer.

In the example of FIG. 1, a LIBS laser 10 directs, its collimatedoutput, when energized by controller subsystem 12, to adjustablefocusing lens 14 which produces a small spot (e.g., 100 μm) of laserenergy on sample 18 creating a plasma. The focusing lens can be moved inthe axial direction, meaning in a direction perpendicular to the surfaceeither closer to or further from the sample as shown by arrow 15.

The resulting photons of the plasma produced by the laser energy proceedalong a detection path including focusing lens 14 to subsystem 20 (e.g.,a spectrometer). The output signal of detector subsystem 20 may beprocessed by controller subsystem 12. In this particular example, highpass filter 21 passes laser energy (e.g., at, for example, 1500 nm) fromLIBS laser 10 to lens 14 and reflects lower wavelengths (e.g., belowabout 1,000 nm) to subsystem 20 which may include a slit.

A translation mechanism 22 may be provided under the control ofcontroller subsystem 12 to move focusing lens 14 in the axial directiontowards or away from the sample surface (vertically in the figure) inorder to permit focusing control for rough sample surfaces as well as tocompensate for any path length variations introduced by the optics. Astepper motor combined with gears and the like can be used to adjust theposition of focusing lens 14. An electromagnetic coil or other means oftranslation may also be used.

Spectrometer 20 may include a CCD detector array as set forth in thedesign of co-pending application Ser. Nos. 13/591,907 and 13/507,654incorporated herein by this reference. Other spectrometers includeechelle (with a 2D CCD), Paschen-Runge, and the like.

Controller subsystem 12 may include one or more micro-processors,digital signal processors, analog and/or digital circuitry or similarcomponents, and/or application specific integrated circuit devices andmay be distributed (e.g., one micro-processor can be associated with thedetector subsystem while a micro-controller can be associated with thedevice's electronic circuit board(s). The same is true with respect tothe algorithms, software, firmware, and the like. Various electronicsignal processing and/or conditioning and/or triggering circuitry andchip sets are not depicted in the figures. Additional optics includingbeam expansion, collimation, and/or adjustment optics are possible insome examples. Beam expansion optic 19 is shown for increasing thediameter of the laser output impinging on focusing lens 14. Laser 10 ispreferably a class 1 eye safe laser.

Mechanism 22 may also be configured to move focusing lens 14 right andleft in the figure as shown by arrow 17 (and/or in a direction in andout of the plane of FIG. 1) to move the laser beam spot to multiplelocations on the sample. In one example, controller 12 is configured toautomatically focus the laser beam on the sample, clean the sample,analyze the sample, and then move the laser beam and again properlyfocus the beam, clean the new location, and again analyze the sample.These features are discussed below.

Another way to move the laser beam to multiple locations on the sampleis to use adjustable optic 16, FIG. 2. Optic 16 may include a tip-tiltmirror electromagnetically or electrostatically driven, MEMS mirrors,and the like such as those available from Mirrorcle Technologies, ThorLabs, Newport, as well as other suppliers.

In FIG. 3, the delivery and return optical paths are similar to thosedescribed for FIG. 1. This example includes a rotating glass window(optic 16) as an alternative method for implementing spot translation onthe sample. The change in refractive index between free space and theglass window combined with the angle of the glass window relative to theoptical axis results in a lateral shift of the laser beam as shown inFIGS. 3A and 3B. Rotating the glass around a glass optic 5 mm thick witha refractive index of 1.5 for example, may be used. If the glass surfaceis angled at 55° to the optical axis, the lateral displacement would beapproximately 2.2 mm. By rotating the glass optic around the opticalaxis as shown in FIGS. 3A and 3B, the focus spot would follow a circleof radius 2.2 mm on the sample surface (e.g., a circle circumference ofapproximately 14 mm). If the glass optic is rotated about the opticalaxis in 6 degree steps, measurements of 60 unique areas of the sampleare enabled each separated by about 0.23 mm.

Another version could include two sequential rotating glass optics,similar to the single optic shown in FIGS. 3A and 3B allowing fulltranslational control in the X and Y directions on the sample ratherthan just being limited to a circle. In still other designs, a compositeglass translation optic could be used to reduce or eliminate refractiveindex dispersion effects which might result in small differences intranslation verses wavelength.

One of the advantages of the geometries of FIGS. 1, 2 and 3 is that theLIBS laser and the optical emission detection optics of the detectorsubsystem stay aligned on the same sample point as the sample locationis modified by the movable optic.

Controller subsystem 12 is typically configured (e.g., programmed) toenergize (e.g., pulse) the laser producing a series of laser pulses andto analyze the sample at one location by processing the output of thespectrometer between pulses. The controller subsystem is typicallyconfigured to receive a trigger signal (generated by the operatorpushing a physical or virtual button or the like) and in response topulse the laser. The controller subsystem then adjusts the movable optic(14, FIG. 1; 16, FIGS. 2-3) and again energizes the laser and analyzesthe sample now at a different location. A typical controller subsystemof a hand held or portable device will typically display, on an outputscreen, the elements detected and, optionally, their concentrations.

Operating the laser in the “eye safe” wavelength range of 1.5 μm offerssignificant advantages to handheld LIBS analyzers. Handheld units are bydesign open beam meaning the laser beam exits the unit before strikingthe sample. Therefore, scattered laser light (or direct laser light inthe case of extreme misuse) could strike the user's eye. However becauselaser light at this wavelength is strongly absorbed by water, the laserlight cannot reach the retina. The laser is therefore rated as either aslow as Class 1 depending on total energy. A Class 1 rating in particularis a significant commercial advantage as it eliminates the requirementof special safety glasses be worn during usage and regulatoryrequirements are greatly reduced compared to the most regulated Class 4type of lasers. An eye safe laser may be preferred (e.g., class 1 or 2)and a safer laser source can be used in some embodiments (e.g., class 3)with the understanding that the class of laser and safe rating dependson variables such as energy level, wavelength, pulse width, pulse rate,divergence angle, and the like.

However, lasers that operate in the “eye safe” wavelength range near 1.5μm create a number of hurdles, addressed below, that are needed to makethis type of laser practical.

The LIBS technique requires that a burst of laser light strikes asample, and deposits enough heat in the area struck so as to generate aplasma. When the plasma cools, electrons from the various elements thatcomprise the sample fall from various excited states to lower energystates, emitting photons in the process. The frequency of the emittedphoton is proportional to the energy of the photon which is, in turn,equal to the difference between the two energy states. The frequency (orits inverse, wavelength) and intensity of the photons are measured by aspectrometer type detector to determine chemical composition of thesample spot where the plasma was created.

Portable or handheld LIBS systems are designed to operate from batteriesand therefore are limited in power. If a portable or handheld LIBSsystem also uses an eye-safe laser, the energy available in the laser,at least with currently available technology, is further reduced. Inorder to generate a sufficient energy density for plasma ignition in thesample region being analyzed under these conditions, the laser ispreferably focused down to a much smaller spot size than required forhigher power bench top lasers, e.g., on the order of 5 μm-100 μm by lens14, FIGS. 1-3. The initiation of a plasma is dependent mainly on powerdensity rather than total power. Therefore, a lower power laser must befocused to a smaller spot size to attain sufficient power density forplasma ignition. It is therefore possible to use a much lower poweredlaser that is more conducive to a handheld or portable LIBS unit and yetstill generate a plasma on the sample surface. The main trade-off oflower power lasers is that the ablation area on the sample will bereduced in area resulting in a more localized measurement and a lowersignal.

A small sample area (5 μm 100 μm in diameter) does however createproblems that should be solved to use a portable or handheld LIBS devicefor real-world applications. First, it can be important that the laserbe focused at the location where the analysis is required. For mostsamples, this is the surface of the sample. A small deviation in thefocus position for whatever reason means the laser is focused slightlyabove the sample surface, yielding incomplete plasma formation, or thelaser light strikes the surface before reaching the focal point (whichtheoretically is at some depth inside the sample in this case). Ineither case, an incomplete plasma is formed with poorer light formationor the plasma is not representative of the sample being tested leadingto erroneous analytical results. Also, in many real-world cases, samplesbeing tested are not completely smooth or they are not flat (such aswires, tubes, rods, etc.). In these cases the ideal focus may vary fromsample to sample such as testing a flat piece of steel followed bytesting a ¼″ diameter steel rod or a ⅛″ welding rod or wire. Adjustablefocusing lens 14, FIGS. 1-3 under the control of controller 12 alsoallows for proper focusing in samples with features which block orinterfere with the head of the portable device.

The second issue is sample cleanliness. LIBS is a very sensitivetechnique and the depth of the region being analyzed is typically justseveral microns, coupled with a sample area diameter of 5-100 μm. It istherefore important that the surface being analyzed is representative ofthe sample and is therefore free of dirt, oils, and/or oxidation. Priorto taking spectral data to determine composition, it is typical to firea number of “cleaning shots” with the laser. These cleaning shots burnoff material on the surface allowing underlying clean material to beanalyzed. However, as stated above in order for the cleaning tests to beeffective, the laser must be properly focused as well. In batterypowered devices, it is important not to fire cleaning shots which arenot required in order to conserve both battery power and analysis time.

A third issue is sample inhomogeneity. For certain types of samples suchas vacuum melt alloys, the samples are likely very homogeneous over a 50μm-100 μm laser beam spot size. However for geochemical samples (soils,sediments, ores) or liquid suspensions (as a few examples), it is likelythat the concentration of the sample changes over a 5 μm-100 μm samplearea. Therefore, it can be important to fire the laser at severaldifferent locations on the surface of the sample and to average theresults.

In embodiments of the invention, translating mechanists 22, FIG. 1-3moves the focusing lens 12. At the first scan location, the laser isfired and a spectrum from the sample is acquired. A typical spectrumthat shows intensity of light measured versus wavelength is shown inFIG. 4. The entire spectrum or one or more regions of the spectrum asoutput by the spectrometer are integrated by the controller 12, FIGS.1-3. The lens 14 is then moved incrementally through a series ofpositions causing the laser focus to occur in front of the sample andthen progressing into the sample bulk. Intensity data is gathered andstored for each focus position. The lens may be moved from a furthestaway position to a closest position (a typical range of about 6 mm) in0.01-1 mm increments.

FIG. 4 shows the intensity data where lens 14 is too far away fromsample 18; FIG. 4B shows the intensity data where lens 14 is too closeto sample 18; and FIG. 4C shows the intensity data when the lens 14 isproducing a preferred, optimum spot size (e.g., 50-100 μm) on thesurface of sample 18. In FIG. 4C, the intensity is at a maximum.Controller 12, FIGS. 1-3, is programmed to detect a maximum or nearmaximum intensity by adjusting the lens focus from outside to inside thesample. The information is then available to the controller on where thelens should be positioned for both large cleaning pulse spots andsmaller spots to be used for data collection.

An example of data from a carbon steel sample is shown in FIG. 5. Theintegrated intensity will approach a peak value at the correct optimalfocal location as shown for position 6 in FIG. 4. For the data shown,the increments were in steps of 600 μm movement for the focusing lens,although the step size can be made smaller. Therefore, when a sample isplaced in front of the device, the first step is that spectra aregathered at several focusing lens locations in order to automaticallyfine tune the focal spot of the laser on the sample. Controller 12,FIGS. 1-3 is configured to perform these steps as part of the initialtesting and to determine and save the focusing lens location thatyielded the maximum intensity. After the optimal focusing position isdetermined automatically, the processor moves the lens to this locationand may then begin testing the sample or optionally, moves the lens tocreate a larger than optimum spot size for the purposes of cleaning,followed by data collection at the optimum (smallest) spot size. Thecontroller is preferably configured to perform this task automaticallyrather than requiring operator input and judgment.

Without a process to automatically focus the laser onto the sample, theoperator may not know if the sample results were correct. Theconcentration results determined by the instrument are related to theintensity of light measured in specific regions of the spectrum. If thelaser is not properly focused, the concentration results will beinaccurate. For a commercially viable product, it is desirable that theinstrument automatically determine the correct focusing location for thelaser. Otherwise, an operator would have to manually performmeasurements to make this determination. This may require a far higherskill level operator and therefore could diminish the commercial successof the LIBS device.

A next step in the analysis is to automatically determine if the samplelocation being tested is sufficiently clean. One cleaning cycle methodis to take multiple repeat laser tests of the area and identify two(typically) of the largest spectra (atomic emission) peaks usingavailable peak finding algorithms. Smaller peaks may also be selectedthat are important to the analysis at hand. These peaks correspond toparticular elements present in the sample area being tested. Additionallaser tests of the sample area are performed. Controller 12 thencomputes a rolling average of the intensity measured for the above twoelements. When the intensity stops changing by less than a predeterminedpercentage from each point in the rolling average (for example by lessthan 5%), then the sample is appropriately cleaned. An alternativemethod for determination of cleanliness would be to compute theintensity ratios of the rolling averages. Once the ratio stabilizes towithin a preset percentage, the sample may be considered to be clean.

An example of peak intensity verses cleaning pulse count is shown inFIGS. 6A and 6B for a sample of rusty carbon steel (photo in FIG. 7).The cleaning cycle requires that a layer of dirt and oxidation be burnedoff by the laser blasts. As shown, the intensity of the carbon peak(FIG. 6A) and iron peak (FIG. 6B) change with sequential tests (laserpulses) until the results approach a stable intensity level. Here, 25laser pulses resulted in a stabilized detector output. The method inthis case may use a five point moving average. The carbon intensity(FIG. 6A) decreases with the sequential cleaning tests ascarbon-containing organic material (i.e. dirt, oils, or skin oils) areburned of the sample. The point at which the carbon intensities stopdecreasing indicate that only the base metal is being tested.

Likewise, the iron concentration (FIG. 6B) increases during the earlycleaning tests as oxidation and other materials are burned off whichwere masking the iron content in the sample. Again, as the change inintensity of the iron photon emissions flatten with increasing testnumber, the base iron in the sample is being analyzed.

In principle, it may possible to only use a single peak for theautomatic determination of when to end the cleaning tests. In addition,when testing for low concentrations of an element, say 1% carbon in 99%iron, the carbon line will be far more sensitive to cleanliness than theiron since the ratio of contamination to sample carbon is large and theratio of contamination to iron is small. The peaks which are selectedfor analysis may include typical elements in the bulk sample or in thecontamination coating such as carbon, oxygen, and silicon. Byautomatically stopping the cleaning cycle when the sample issufficiently clean, battery power is conserved and testing time isreduced. It should be noted that the process of finding the optimalfocal length for the sample, described above, also provides somecleaning of the sample spot, thereby reducing the number of cleaningtests performed in this step.

One preferred cleaning method also results in an optimal manner toperform the cleaning and the subsequent sample analysis. Based on thetesting performed to develop this method, a number of observations weremade about the sample cleaning. Consider the pictures of a sample shownin FIGS. 7A and 7B. Upon examination of the area struck by laser, it wasobserved that the inner portion of the circular laser spot area 40, FIG.7B, is well-cleaned but near the perimeter 42 of the analysis area, thecleaning may be less thorough.

In the real world of non-ideal lenses, lasers, and diffraction limitedoptics, it is expected that the inner component of the laser beam willdeliver more energy to the sample than the outer perimeter of the beam.The region of the sample will thus be better cleaned more towards thecenter of the sample area. Therefore, an additional embodiment of thecleaning cycle method is to clean a larger area, in one example, than isactually analyzed. After the controller determines the optimal laserbeam focal length as described previously, the focusing lens is movedsuch that the beam striking the sample surface during cleaning tests isabout 20% larger. See, e.g., FIG. 4B. When the controller determinesthat the sample region is adequately cleaned, according to the abovedescribed steps, then the controller returns the focusing lens to theoptimal position previously determined and stored. This assures that thearea struck by the laser during analysis is therefore smaller than thearea cleaned assuring that the area to be analyzed is thoroughlycleaned.

Another problem addressed is sample non-homogeneity. Many samples, forexample geochemical samples encountered in the analysis of soil, ores,sediments and/or slurries are not homogeneous across the sample face. Inother techniques, such as x-ray fluorescence analysis, the samples arecollected and ground to about a 100 μm particle size prior to analysis.However, 100 μm is approximately the same size as the laser beam on thesample in the case of a LIBS analysis in accordance with embodiments ofthe invention. It is therefore desirable to test multiple locations onthe sample and average the results.

The method provides for an optical/mechanical means which moves thelaser beam spot across the sample as discussed with respect to FIGS. 1-3to address the problem of non-homogeneous samples. FIG. 3, for example,shows an optical component 16 that is angled with respect to the laserbeam striking it. As the optical component rotates by discrete amounts,the laser beam is directed to different locations on the sample.Therefore, a preferred method used locates the laser beam at aparticular spot on the sample, finds the correct focal length bytranslating the focusing lens 14, and then performs the cleaningoperations as described above, followed by the sample analysis. Theoptical component 16 is then rotated a discrete amount, for example 60degrees, to yield a different sample location which it is also cleanedand analyzed. At each location, the optimal focus is determined andsaved for the laser spot on the sample as described above. In a furtherembodiment, if the analytical results (e.g., the concentration of thetop five elements changes by 10% or less) for a second or third testinglocation are not appreciably different than the first testing location,the controller terminates the measurement process and the controlleraverages the results. Thus, for homogeneous samples, only two to threelocations are cleaned and analyzed conserving power in a batteryoperated device. For non-homogeneous sample, five locations may becleaned and analyzed. The controller is preferably configured to reportto the operator when the sample is homogeneous and/or non-homogeneous.Note that XRF techniques are not able to determine if a sample isnon-homogeneous.

FIG. 8 depicts the processing of controller 12, FIGS. 1-3 in onepreferred embodiment. The focusing cycle is initiated, step 60, inresponse to a trigger signal followed by the cleaning cycle, step 62,for each sample location. These cycles may be reversed. At each locationon the sample, the spectrum analysis is performed, step 64, wherein theelemental concentrations are computed, reported, and typically saved.The hand held portable unit, see FIG. 12, preferably has a displayscreen for displaying the elements present in the sample, theirconcentrations, and other data. In general, the controller subsystem isconfigured, (e.g., programmed) to pulse the laser producing a series oflaser pulses and to process the resulting signals from the detector(spectrometer) subsystem to determine one or more elementalconcentrations in the sample. For LIBS analysis, the detector outputssignals representing intensities at different wavelengths defining theelements in the sample and the various concentrations.

The laser beam spot is then moved, step 66 whereupon the focusing,cleaning, and analysis cycles repeat for the new sample location.Sequential locations are thus analyzed.

In the focusing cycle, the controller is configured to adjust thefocusing lens, step 70, pulse the laser, step 72, and analyze theintensity data reported by the detector electronics, step 74 (see FIGS.4A-4C) until an optimum intensity is detected (which is at or near themaximum), step 76. The lens position which resulted in the optimumintensity is stored, step 78. A memory accessed by the controller may beused to store lens position values, calibration constants, spectraldata, algorithms, computer code, and the like. FIG. 5 demonstrate anoptimum focus location in the range of positions 4 to 7.

In the cleaning cycle, FIG. 10, the focusing lens is moved to theoptimal position, step 80, or optionally moved to produce a slightlylarger spot size, step 82. The laser is repeatedly pulsed, step 84, andfor each resulting plasma, one or more peaks are analyzed, step 86. Thecleaning cycle stops: when the intensity data indicates the intensityhas stabilized, step 88 and 90 as shown in the example of FIGS. 6A and6B.

The moving spot cycle, FIG. 11, preferably includes running the focusingcycle, step 60 and running the cleaning cycle 62 at each location on thesample. At the optimal laser spot size, the laser is pulsed, step 92 andthe spectrum is analyzed, step 64. Typically, a minimum number of samplelocations are tested (e.g., 3), step 60 as depicted at 94 and if theminimum number has not been reached, the movable optic (14, FIG. 1, 16,FIGS. 2-3) is adjusted to move the beam to a different location on thesample, step 96, FIG. 11. The focusing, cleaning, and analysis cyclesare again repeated for this new sample location until the analysis asbetween different sample locations indicates a homogeneous sample asshown at 98 or a maximum number of sample locations (e.g., 5) have beentested, step 100 (for non-homogeneous samples). Alternatively, once thesample is determined to be non-homogeneous, a predetermined number ofnew sample locations may be analyzed. Preferably, the results areaveraged for both homogeneous and non-homogeneous samples and reported,step 102.

The number of required sequential sampling locations may depend on howheterogeneous the sample is. It is desirable to minimize the requiredsampling time, so various algorithms may be employed as data iscollected to optimize the sampling time required. One algorithm startswith a minimum sampling location count (3 locations for example) toestablish a baseline variance or standard deviation in constituentconcentration. If the standard deviation is above a pre-set threshold,then the algorithm will initiate further measurements from additionalsample N locations.

Each time a new location is sampled, the standard deviation of the dataset is calculated. The precision of the mean (or average) concentrationis related to the standard deviation and the number of samples N in thedata set by:

$\begin{matrix}{\sigma_{mean} = {\frac{\sigma}{\sqrt{N}}.}} & (1)\end{matrix}$

The “standard deviation of the mean” is a measure of how stable thecomputed average of the measured concentrations are. The algorithmterminates further sample location measurements once the standarddeviation of the mean is below a pre-set threshold. Often with suchalgorithms, a maximum sample location count is programmed to force theinstrument to stop measuring after a certain time limit is reached. Suchalgorithms can also make estimates of time to completion based on therate of improvement of the “standard deviation of the mean” (orsimilarly computed indicator) after the first several measurements. Theuser may be given the option to wait for completion or to stop themeasurement.

FIG. 12 shows a handheld portable unit housing the subsystems andcomponents of FIGS. 1, 2, and/or 3 and with the associated electroniccircuitry carrying out the analysis, signal processing, and controlsteps depicted above with respect to FIGS. 4-6 and FIGS. 8-11.

An argon purge subsystem may be included for better analysis of thesample for certain elements including sulfur, phosphorous, and/orcarbon.

In some embodiments, the focusing lens adjustment cycle is performedwithout moving the laser spot to multiple locations on the sample andvice versa. The cleaning cycle is, in some embodiments, preferred and inanother aspect is optional and/or separately patentable.

In one preferred embodiment, the hand held LIBS spectrometer is batterypowered and employs an eye safe laser. The automatic focusing stepsensure repeatable, more accurate elemental concentration results withoutoperator intervention. Automatic focusing provides more repeatableresults, without operator intervention, and more accurate results.

The cleaning cycle ensures that the laser adequately cleans the samplewhile at the same time saves testing time and battery power because,once the sample is adequately cleaned, no more cleaning laser pulses areneeded. This reduces the number of laser shots and therefore makes thetest conclude faster and saves battery power.

Adequate sampling of all samples is performed and battery power andtesting time are conserved.

FIG. 12 shows one example of a battery powered, portable LIBS analyzer200 with gas (e.g., argon) cartridge 202 loadable therein. As shown inFIG. 13, one or more nozzles 204 a is fluidly connected to cartridge 202via valve 206, FIG. 12. In other examples, a cartridge or small tank isconnected to unit 200 and carried in a small pack for field analysis.

Preferably, only a small supply of argon is required in the purgesubsystem because the nozzle(s) is configured to deliver a small sprayof argon gas locally in a small purge volume. Unit 200 may have aconverging front nose 210 where the laser beam exits to strike sample212 at location 214 (e.g., 5-100 μm in diameter) creating a plasma 219.Nozzle 204 a is just inside distal nose 210 proximate end wall 216 andoriented to produce an argon spray at (and preferably only at) location214. The nozzle has an orifice configured to produce a purge volume ofargon gas less than 1.0 cm³, typically as small as 0.5 cm³ as show at218 so it just surrounds the plasma 219 and little argon is wasted. Inone example, the argon gas volume was 0.125 cm³. As discussed below, theflow rate is low and the argon purge is used only when needed in orderto further save argon resulting in a LIBS analysis unit which does notrequire a large supply of inert gas.

FIG. 14 shows controller 12 controlling solenoid valve 201 betweensource 202 and nozzle 204 a. A trigger signal as shown at 220(generated, for example, by pressing on trigger mechanism 222, FIG. 12)is received at controller 12 and, in response, controller 12 mayoptionally initiate the cleaning cycle as discussed above. Anothertrigger mechanism may include a physical or virtual button.

During the subsequent analysis cycle, controller 12 opens valve 206 justprior (e.g., 0.1-0.5 seconds before) the first plasma producing laserpulse as shown in FIG. 15. FIG. 16 shows a strong signal for carbon in atest sample even when the purge occurred just 0.1 seconds prior to thefirst laser pulse. Controller 12, FIG. 14 is further configured to closevalve 206 shortly after the last laser pulse or even prior to the lastlaser pulse as shown in FIG. 15 in order to conserve the purging gas.

FIG. 16 depicts the influence of the flow rate, nozzle position, andpurge timing on the resulting LIBS signal output by spectrometer 20,FIG. 14. In test A, no purge was used and the carbon peak was difficultto correctly decipher. In test B, the purge rate was 4 CFH, the nozzlewas 0.2 cm away from the sample and the plasma location, and the purgegas was initiated 0.5 seconds before the first laser pulse. In test C,the nozzle position and the flow rate were the same as in test B but nowthe purge gas was initiated only 0.1 second before the first laserpulse. The signal strength was still very high. In test D, a lower flowrate of 0.5 CFH was used and the purge occurred 0.5 seconds before thefirst laser pulse while in test E the lower gas flow rate of 0.5 CFH wasused and the solenoid valve was opened for a purge 0.1 seconds beforethe first laser pulse. In both cases, the signal strength wassufficiently high. In test F and G the nozzle was brought closer to thesample (0.1 cm away from the sample). In test F a flow rate of 4 CFH wasused with a 0.5 second purge delay and in test G a 0.5 CFH flow rate wasused with a 0.5 second purge delay.

Accordingly, it is possible to use a very low flow rate of 0.5 CFH and avery short (0.1 second) delay before the first laser pulse and stillobtain a sufficiently strong signal from the resulting photons. A purgerate of less than 2 CFH may be optimal.

In one typical scenario, the output of spectrometer 20 is analyzedbetween the laser pulses shown in FIG. 15. In some examples, if thevalve can be actuated at a high frequency rate, the gas can even beturned off between laser pulses and then on again just prior to eachlaser pulse.

Thus, in one preferred embodiment, an improved signal is generated anddetected by the spectrometer using an inert gas purge. The gas isconserved by using a low flow rate and a smaller size nozzle properlylocated and oriented to produce a small volume purge spray. And, thepurge is used only when required. One result is the ability to use onlya small cartridge as opposed to an unwieldy tank in a portable, handheld, battery powered system. When one cartridge is emptied, anotherfull cartridge can be quickly loaded into the unit.

In one embodiment, handheld LIBS spectrometer 300, FIG. 17 includes aneye safe laser (e.g., Kigre MK88) and functions to automatically focusthe laser beam, move the laser beam focus, clean the sample using themoving laser beam, and auto-calibrate.

Shown in this particular example is housing 302 roughly in the shape ofa pistol like device with rechargeable batteries 304 a and 304 bproviding power to the subsystems of the device. The top portion of thehousing may be made out of aluminum for laser heat dissipation andbottom half may be plastic. Handle or grip portion 308 includes areplaceable gas (e.g., argon) cartridge 310 therein. Printed circuitboard 312 may include the necessary processing, signal conditioning,power supply, motor control, and other circuitry as described herein.Addition circuit boards may be included. Printed circuit board 312 mayinclude one or more field programmable gate arrays programmed as setforth herein.

Trigger 314 is used to actuate a laser inside laser enclosure 360 fixedto housing 302. LCD display 316 may be included at the rear or top ofthe housing to display the results of an analysis, to enter commands,and the like.

FIG. 18 shows optics stage 320 including optics head 322 mounted in theforward section of the housing and including laser focusing lens 324mounted in laser beam channel 326 and detection lens 328 mounted indetection channel 330. Camera 332 and camera lens 334 may also memounted in optics head 322. The optics stage is configured to move theoptics head relative to the housing. The laser source provides a laserbeam depicted at 336 focused by lens 324 and directed to and exiting anaperture 338 in nose section 339 end plate 340 defining a minimum volumepurge chamber 341 with optically transparent shield 342 disposed betweenoptics head 322 and purge chamber 341. The transparent optical shield infront of the laser focusing lens 324 and detection lens 328 may be madeof fused silica, quartz, or sapphire for example and functions toprotect laser focusing lens 324 and detection lens 328 of the opticstage, which are very close to the front end of the device, from plasmagenerated by the laser exiting end plate 340 orifice 338 abutting asample to be analyzed. End plate 340 and/or shield 342 can be removedfrom the housing (by removing fasteners such as screws) and cleaned orreplaced by the user as necessary when shield 342 becomes dirty, pitted,or the like. The optical shield also functions to create a small chamberfor argon containment. It also serves to protect the optical elementsfrom the outside environment.

Purge chamber 341 is preferably purged with an inert gas such as argonas discussed herein supplied by cartridge 310, FIG. 17.

The laser beam 336, FIG. 18 is focused to a small spot size by lens 324onto a sample adjacent end plate 340. Typically, end plate 340 is placedon the sample to be analyzed. The resulting plasma and electromagneticradiation emitted from the plasma passes through aperture 338 and isfocused by detection lens 328 into detection channel 330. A wave guidesuch as a fiber optic bundle may be coupled to detection channel 330 atport 350 and couples the electromagnetic radiation to a spectrometersubsystem preferably including an array of individual spectrometerenclosures as discussed herein. The spectrometer subsystem provides anoutput to the controller subsystem which controls the firing of thelaser source and commands one or more motors configured to advance andretract optics stage 332, to move it left and right, and/or to move itup and down. In this way, the laser beam can be automatically focused,can be used to clean the sample by moving the laser beam about thesample, can be used to take readings at different locations on thesample, and to auto-calibrate the system.

Beam translation on the sample surface makes use of the property of afocusing lens 324, FIG. 19 to focus all incoming parallel light rays toa single point collinear with the center of the lens: A collimated beamof light (parallel light rays) entering the lens is focused to a point.Collimated beams of smaller diameter are still focused to a point at thesame location. Moving the beam relative to the lens while keeping itparallel to the lens optical axis does not change the location of thefocus.

If, however, the beam is kept stationary and the lens 324 is movedrelative to the beam, the focused spot will also move relative to thestationary collimated beam. When the focusing lens is translated normalto the collimated beam, the focused spot will move in exactly the samedirection and degree normal to the collimated beam. The focused spotwill always be on the optical axis of the focusing lens. Detectionoptics 328 directed at that same point on the focusing lens optical axisand which is moved along with the focusing lens will thus be able toexactly track the beam as it is moved around on a sample surface.

The detection optics 328 are directed at the same point to collect lightfor spectroscopic measurement and focus the electromagnetic radiationresulting from the plasma generated by the laser spot on the sample.FIGS. 19A and 19B show how both the focusing lens and collection opticsare translated normal to the collimated beam (downward in the diagram)while the collimated beam and sample remain stationary. The focus anddetection optics are both translated to move the laser spot from point Ato point B on the sample.

A system of guide rails, bearings, rotary motors, and threaded driverscan be used to move the optics head 322 relative to the housing. In oneparticular example shown in FIGS. 20-22, the optics stage includesmoveable optics 322 head in front of laser source 360 and first Z-shapedframe member 362 moveably coupled to the housing via plate 364, FIG. 20and slideable right and left via linear bearing 366 attached to frame362 and rail 368 attached to plate 364. There are means for moving theoptics head. In one example, motor m₁ is attached to plate 364 withshaft s₁ driving nut n₁ engaging or formed in first frame 362 as shownat 340. This arrangement moves first frame 362 and the optics head 322right and left relative to the housing and the nose section thereof.

U-shaped frame member 370, FIG. 20 moves up and down relative to framemember 362 via linear bearing 372 attached to frame member 362 and arail 374 attached to frame member 362. Motor m₂ is fixed to frame member362 and shaft s₂ thereof drives nut n₂ engaging frame member 370. Thisarrangement moves frame member 370 and optics head 322 up and downrelative to the housing and nose section.

As shown in FIG. 22, rail 380 is attached to optics head 322 and linearbearing 382 is attached to frame member 370. Motor m₃ is attached toframe member 370 and shaft s₃ thereof drives nut n₃ engaging optics head322 for advancing and retracting the optics head in the direction of thelaser beam from the laser source relative to the housing and nosesection thereof (z-direction).

Such movements under the control of the controller subsystem can be usedto autofocus the laser beam. Movement of the optics head up and down andleft and right can be used to automatically clean the target sample,move the focused laser beam about the sample for measurement at multiplesample positions, and calibrate the system. Other types of motors andbearing may be used including linear motors, piezo motors fluidlyactuated motors, and the like. Other frame or stage members arepossible. In other embodiments, the optics head has only one or twodegrees of freedom.

In some embodiments, laser focusing lens 324 can be moved (e.g.,translated) independently of detection lens 328 by incorporating a motorfor laser focusing lens 324. A motor drive similar to that used in acell phone ca era zoom lens can be used. See U.S. Pat. No. 7,309,943incorporated herein by this reference. The detection lens may be movedindependently as well using a similar motor.

In one example, the end plate 340, FIG. 18 is made of or includes aportion 343 made of a calibration standard material e.g., 316 stainlesssteel. The presence of more than one material is also possible for moreprecise calibration over large wavelength ranges. The optics head ismoved automatically to aim the laser beam at portion 343 andautomatically focus it there to a small spot size. Electromagneticradiation from the resulting plasma is received at optic lens 328 indetection channel 330 and processed by the spectrometer subsystem andcontroller subsystem for calibration purposes. Then, the optics head maybe automatically moved to aim the laser beam at the sample throughaperture 338, to focus the laser beam on the sample, to clean it, andthen to test the sample at various locations. Preferably, autofocusingoccurs at each sample location to account for surface irregularities,dimples, crevices, and the like.

FIGS. 23-24 show different views of the frame members, motors, bearings,and the like. Also shown are sensors (e.g., pogo pins) p₁, p₂,configured to sense the end of travel of the two frame members and theoptics head. For example, pin p₂ is used to detect the end of travelleft position of frame member 362 while pogo pin p₁ is used to detectthe end of travel rearward of the optic stage. Additional pogo pins, allproviding an output to the controller subsystem, are used to detect endof travel right and left, up and down, and forward and rearward of themoving components so that controller subsystem and/or motor controllerscan properly control motors m₁, m₂, and m₃ in a way which prevents anyshock being applied to optics head 322.

FIG. 25 shows fiber optic bundle 400 coupled to optics head 322detection channel port 350 with a plurality of optical channels such asfiber bundles 402 a-402 d terminating in couplers 404 a-404 d.

Each coupler is coupled to a spectrometer enclosure 406 a-406 d, FIG. 26sandwiched between printed circuit boards 312 and 313 in housing 302behind laser source enclosure 360 and in front of LCD display 316. Allthe spectrometer enclosures are preferably coupled together.

As shown in FIG. 27, each spectrometer enclosure 404 is thin (e.g.,0.5″) and preferably about 4″ long and 3″ wide defining a floor 420 andfour side walls 422 a-422 d and an open top with optical devices withincavity 424. The thickness of the enclosure is dictated by the tallestoptical component therein.

In this way, to conserve space in a handheld device, the outside of thefloor of one enclosure forms the top cover of an adjacent enclosure whenthe enclosures are coupled together via fasteners through enclosurecorner holes such as shown at 426. The result is a more compact design.The structural configuration of each spectrometer enclosure is typicallythe same aside from differences such as grating, grating angle and inputfilter. This feature also reduces manufacturing costs. The finalspectrometer enclosure in the array will typically include a lid 428,FIG. 26 covering its open top. The multiple spectrometers in an arrayprovide a wide range of wavelengths and good resolution.

In one design, each spectrometer enclosure has optical devices in thedesign of a Czerny-Turner configuration. Here, electromagnetic radiationdirected to an optical fiber 402 bundle from the detection channel ofthe optics head (resulting from the plasma generated by the laser beam)is redirected by mirror 440 which collimates the light and directs it tograting 442. The different wavelengths diffracted off of the grating aredirected to mirror 444 which focuses the radiation on sensor 446 (e.g.,a CCD sensor chip). The output of the CCD sensor is then provided to thecontroller subsystem for further processing, display, elementalanalysis, and categorization by reference to spectrum referencelibraries stored in memory, and the like.

Preferably, optical adjusters are provided where required, between theparticular optic mount and the housing, to allow for precise alignmentof the dispersed optical wavelengths to the CCD detector. For example,mirror 440 is mounted to fixture 448 coupled to but adjustable withrespect to spectrometer enclosure floor 420. The angle of mirror fixture450 can be adjusted via a set screw at 452 and mirror fixture 450 can betilted via shaft 454. Grating fixture 456 can be adjusted via fasteners458 and 460 received in spectrometer enclosure floor 420.

To tailor each spectrometer enclosure 406 a-406 d, FIG. 26 to a specificwavelength range, each enclosure may include different gratings and/orgratings disposed a different angles. The result is a handheldspectrometer which can detect a wide variety of elements. For LIBSapplications, it is desirable to have a wavelength resolution ofapproximately 0.1 nm over a wavelength range from 175 nm to over 675 nm,a range of 500 nm or more. To cover 500 nm with 0.1 nm resolution, aminimum of 5000 pixels is required, ideally 50% more. Typical CCD pixelcounts are 2048 to 3600. In this example, four spectrometers areemployed utilizing 2048 pixels per spectrometer yielding just over 8000total pixels.

As discussed above, gas cartridge 310, FIGS. 28A-28B is preferablydisposed in handle section 308 behind door 472 removably coupled toregulator 470 itself rotationally coupled to the housing. In this way,the gas cartridge can be pivoted out of the handle for replacement.Regulator 470, in turn, is fluidly coupled to a valve such as solenoid480, FIGS. 17 and 24 which is controlled by the controller subsystem.Solenoid 480, in turn is fluidly coupled to purge chamber 341, FIG. 18between end plate 340 and shield 342. The only opening into purgechamber 341 is via aperture 338 in end plate 340 to conserve purging gasand preferably to eliminate the need to couple the handheld device to alarge gas tank.

FIG. 29 shows batteries 304 a and 304 b in handle 308 base 490 withincover 492 allowing replacement of the batteries. The batteries can alsobe recharged while residing in base 490 using known rechargingreceptacle designs and the like.

The controller subsystem 12, FIG. 30 is configured to control laser 315,valve 480, motor controllers 500 for optics stage 320, and display 316based on, for example, signals output from spectrometer subsystem 20 andtrigger 314.

Preferably, controller subsystem 12 controls motor controllers 500 tomove optic stage and thus laser focusing lens 324 (and detection lens328) forward and rearward to create a small (e.g., 5μ-100μ) spot sizefocused automatically by reading the output of spectrometer subsystem 20as the laser is automatically fired by controller subsystem 12 and byanalyzing one or more element intensity peaks as discussed above withreference to FIGS. 4A-4C. Controller subsystem 12 may be configured toperform this autofocusing step as a part of initial testing and todetermine and save the focusing lens location that yielded the maximumintensity for one or more elements. Controller subsystem 12 may furtherbe configured to automatically trigger laser 315 to fire a number ofcleaning shots to burn off material on the surface of the sampleallowing underlying clean material to be analyzed. By controlling themovement of the optics stage, the cleaning shots are focused and can bedirected to different portions of the sample. Controller subsystem 12may be configured to automatically focus the laser beam by moving opticsstage 320 to position the laser beam at a number of different areas onthe sample and to automatically detect and terminate the cleaning shotsto save battery power when the sample is sufficiently clean foranalysis.

Controller subsystem 12 may further be configured to control laser 315and optics stage 320 to move the laser beam among several differentlocations on the sample or target surface to enable an appropriatedetermination of sample homogeneity and to automatically stop the laserfiring sequence to save battery power once it is determined that theappropriate spectra have been detected and/or the sample is homogeneous.

To detect certain elements, gas purging may be required and controllersubsystem 12 controls valve 480 to provide purging gas to nose section339. Preferably, only a small supply of purging gas is required in thepurge system because one or more nozzles is configured to deliver asmall spray of argon locally to the nose section purge chamber.Controller subsystem 12 may also control valve 480 as a function of thelaser firing sequence to further limit purging gas use during analysis,cleaning, calibration, and the like as set forth above.

In some examples; a microprocessor(s), computer, application specificintegrated circuit, field programmable gate array, computer server andclient subsystem, or similar means is programmed as set forth herein.Other equivalent algorithms and computer code can be designed bysoftware engineers and/or programmers skilled in the art using thespecification provided herein.

As shown in FIG. 31, the auto-calibration routine carried out by thecomputer instructions operating on controller subsystems 12, FIG. 30includes instructions which first determine whether calibration isneeded, step 510. Auto-calibration can be initiated based on a triggersignal generated by trigger 314, FIG. 30, based on a command signal (forexample when the user uses display 316 to select “auto-calibrate”), by alapse of time (e.g., one to two hours since the last calibration), by atemperature change (e.g., as detected by temperature sensor 481, FIG.30), or based on other criteria.

If calibration is determined to necessary or desirable, the optics stageis controlled to move the optics head and laser beam to presetcoordinates so the laser beam focuses on the calibration standard 341,FIG. 18 of end plate 340, step 512, FIG. 31. These calibrationcoordinates may be stored in a database 514 (in memory, for example suchas a RAM, ROM, PROM, or the like).

The laser is then powered providing a laser beam forming a plasma at thecalibration standard, step 516 and spectral data is collected via thespectrometers and the resulting spectrum is analyzed, step 518 againusing equations, spectral libraries, and the like in database 514. Ifneeded, due to temperature changes, aging of components of the system,spectral drift of a spectrometer, or the like, the stored intensityand/or wavelength calibration constants and/or calibration curves can beverified, adjusted, or changed, step 520 for more accurate analysis whenthe device is subsequently used to analyze a sample.

In an analysis, the process typically begins when the unit is powered onand/or a trigger signal is received, step 522, FIG. 32. Theauto-calibration routine of FIG. 31 may be carried out if necessary,step 524. If the temperature as sensed by the temperature sensor iswithin a specified range and a calibration has been carried outpreviously within a specified time limit, then auto-calibration is notneeded. Then, the optics head is moved to produce a laser beam at onespot on the sample and the auto-focus routine of FIG. 9 is carried out,step 526, FIG. 32. Then, or concurrently with auto-focusing, cleaningshots may be generated using the auto-clean sequence depicted in FIG.10, step 528, FIG. 32. In some examples, cleaning is effected by firinga predetermined number of laser pulses (e.g., 5-10). For the analysis ordata shots after cleaning terminates, step 34, purging may begin asshown at 536. After each pulse of the laser, the spectral data of theresulting plasma is collected and analyzed (preferably using theadjusted calibration constants), step 538. Next, purging may terminateto conserve the purge gas as shown at 540 and the optics head is movedto produce a laser spot and to sample/test at a new location on thesample, step 542. The various locations may be pre-programmed.

Then, the auto-focus, auto-clean, data shots, and spectral analysis stepmay again be performed at the new location. Due to the optics stage,spots in a circle, square, cross, X, or other shape can be generatedsince the stage moves left and right and up and down relative to thesample. In some embodiments, at a new location, one or more cleaningshots may be performed first. If the intensity of the plasma detected issimilar to the last spot, it may be assumed the laser beam is properlyfocused, so the auto-focusing steps may be skipped.

In some examples, this sequence terminates after a preset number oflocations have been analyzed. In other examples, this sequence continuesuntil it is determined the sample is homogeneous as shown in steps94-100, FIG. 11. Pulling on the trigger may also terminate the testing.When the testing is complete as shown at 544, FIG. 32, the analysisresults (elements present in the sample and concentrations of saidelements) may be displayed on display 316, FIG. 17 and/or stored, step546, FIG. 32.

FIG. 33 shows the primary electronic components of the systemconstituting the controller subsystem 12, FIG. 30. One printed circuitboard 312, 313, FIG. 26 typically includes a LIBS processor module 600,FIG. 33, and the other printed circuit board includes laser driver board604. LIBS processor module 600 is shown in FIG. 34 and includes or isembodied in a system on module card 602. Laser driver board 604, FIG. 33includes microprocessor 620 and the other circuitry shown in FIG. 35.

Processor module 600 and laser driver board 604 may work together tocontrol laser 360, FIG. 33, XYZ optics stage 320, solenoid 480, camera332, and camera LED 331. Signals from the CCDs 446 of spectrometers 406and camera 332 are typically provided to LIBS processor module 600 andprocessed thereby. Signals from temperature sensor 481 and pressuresensor 483 may be provided to laser driver board 604 and processedthereby.

In one example, a trigger signal from trigger 314 is provided to LIBSprocessor module 600 via laser driver board 604 and laser processormodule 600 determines if battery power, the temperature of the unit,and/or other conditions allow firing the laser. If the laser is to befired, LIBS processing module 600 signals laser driver board 604 tocontrol the firing of laser 360 via the system on module card 602 FIG.34 (e.g., an android processor module) and field programmable gate array(FPGA) 608.

Signals relating to electromagnetic radiation captured by the four CCDs446 (FIG. 27) are delivered as shown at 609 to FPGA 608 via AIDconverters 610 and 611 and to system on module 602. System on module 602via FPGA 608 also controls CCD clocking and power as shown at 612. Inthis way, data collection between each firing of the laser isaccomplished in an expedited manner.

When trigger 314, FIG. 33 is actuated, a signal is provided to laserdriver board 608 which sends a signal to system on module 602, FIG. 34located on laser processing module 600. System on module 602 sendssignals to laser driver board 604 microprocessor 620, FIG. 35 whichcontrols the motor drivers as shown at 630 to focus the laser beam to anew sample position. System on module 602 then issues a sequence ofcommands to FPGA 608 to perform a sequence of laser firing shots (forexample 10). Data is collected from each resulting plasma between shotsvia output from the CCDs as shown at 609 in FIG. 34 provided to systemon module 602 via FPGA 608 and then analyzed by system on module 602 forsignal quality. Depending on any thresholds, system on module 602 mayissue commands to FPGA 608 and microprocessor 620, FIG. 35 to collectfurther data from the same location on the sample. Or, system on module602 may issue commands to move the laser beam to a new sample positionand repeat the process described above.

Included within the laser firing sequences above, system on module 602may issue commands to microprocessor 620, FIG. 35 of laser driver board604 to command microprocessor 620 to measure the gas pressure as outputfrom pressure sensor 483. If the gas pressure is above, a presetthreshold, microprocessor 620 may control solenoid 480 to begin gaspurging prior to firing the laser 360 and to stop the flow of gasimmediately after the last shot by controlling microprocessor 620controlling solenoid driver 480. If adequate pressure is not sensed,system on module 602, FIG. 34 may display a message to display screen316, FIG. 33 indicating the need to change out the gas cartridge.

During on measurement periods, system on module 602 is programmed tocollect information from the laser driver board relating to batterycharge, argon pressure, laser temperature, general internal temperature,and multiple internal power rail levels. If anything falls outsidepreset thresholds, appropriate actions can be taken such as flashing abattery level to warn the user that the batteries are low, on display316, to prevent measurement if the laser is too hot, to preventmeasurement if argon is in use and there is no argon left, and providewarnings if internal power rail voltages are outside acceptable ranges.The user may view the sample area on display screen 316 and record apicture with via on board camera 331.

Also shown in FIG. 33 is mini USB “on the go” connector 700, LCD displayand touch screen 316, battery charger PCB 704 that controls batterycharging with dual battery balancing during charging and discharging,status LED 708 located in the power button to indicate unit us poweredup, and Power button 710, used to turn the unit on and off. FIG. 34 alsodepicts GPS module. 712 for determining device location andcommunicating such information to module 602, and Wifi module 714 usedto connect to and communicate with local WiFi networks.

Platform flash memory 716 is used to contain FPGA code that is loaded tothe FPGA on power up. Flash 716 may be reprogrammed by the SOM 602 asneeded. SD card 718 is used for saving collected data. USB Host port 720is an extra port available as needed. Audio Monocodec 722 is used toconvert SOM digital data into analog drive signals to the speaker 724.This can be used to announce material identification or other possiblewarnings to users. Serial “debug” port 726 is used at the factory forlow level communication with the SOM 602. Accelerometer 728 can be usedto determine instrument orientation so that the LCD display may beproperly rotated and also used to record extreme shock experienced bythe instrument.

FIG. 35 shows various connectors 730 from input arrows to named devices,six volt power supply 740 used by multiple sections on the board and offboard, 3.3 volt power supply 742 used by multiple sections on the boardand off board, connector 744 to the power on button, connector 746 tothe trigger button, and 748 connector to the battery charger board.

FIG. 36 shows nose end plate 340 placed on a sample S to be analyzed.Laser aperture 338 may be blocked by the sample and thus purge gasdelivered to purge chamber 341 below shield 342 (transparent to bothlaser and LIBS emission wavelengths) may be prevented from exiting laseraperture 338. More gas may be required and/or the purge gas may not flowoptimally about the resulting plasma.

In order to deliver fresh purge gas to the plasma region, the new designof FIG. 37 is preferred in one particular version of the handheld LIBSanalyzer. Here, end plate 340′ front face 820 includes a vent forremoving purge gas from the purge cavity when the end plate is placed ona sample. In this particular example, the vent has the form of a channel822 in the front face 820 of end plate 340′. Channel 882 extends acrossthe end plate from one side thereof to the other on opposite sides oflaser aperture 338. Gas now flows out of the aperture 338 and withinchannel vent 822 to improve purging of the plasma region without usingexcess purge gas. Fresh argon, for example, is thus provided to thepurge cavity and to the region where the plasma forms on the sample.

FIG. 38 shows the rear of end plate 340′ and purge cavity 341′ formed byrearwardly extending enclosure 830 having in this particular example,four walls 832 a-832 d each with a top rim 834 a-834 d for seating theshield thereon to cover purge cavity 341′.

Purge gas enters purge cavity 341′ via a side opening 836 a, 836 bconfigured here in enclosure side walls 832 d and 832 b so removable endplate 340′ can be installed in either orientation on the nose section ofthe handheld LIBS analyzer using fasteners received through holes 840 aand 840 b in the end plate. A calibration standard (see FIG. 8) may bereceived and secured in each of depressions 842 a and 842 b in the rearsurface of end plate 340′. The two calibration standards are thuslocated on opposite sides of laser aperture 338.

In other examples, the material of end plate 340′, FIG. 38 serves as thecalibration standard. In one example, the end plate was made ofstainless 316 or aluminum 7075. One benefit of depression 842 a and 842b is to make the internal calibration area of the end plate thin. Thisreduces the required travel of the optics stage in the z-direction inorder to focus the laser beam on the internal calibration standard.Also, the risk of creating a plasma is lowered on a dirty shield 342,FIG. 18. The material chosen for the calibration standard should haveemission peaks at the end of each spectrometer range so wavelengthoffset and scale expansion can be analyzed during calibration for eachspectrometer in the stack.

In one example, the auto focusing routine described above is performedon a calibration standard as the X-Y stage orients the laser beam tostrike the calibration standard and then the auto focus routine iscarried out by moving the Z-stage up and down. Then, the distance to thesample is known and the 2-stage can be adjusted to properly focus thelaser beam on the sample since the distance between the calibrationstandard and the sample is known.

The X-Y optics stage then moves to deliver the laser beam throughaperture 338. It may not be necessary to refocus the laser beam forsubsequent sample locations. And, in one embodiment, the X-Y opticsstage automatically moves the laser beam to fire one analysis shot atmultiple locations in an array type pattern on the sample. Cleaningusing laser beam may not be required at each location or at all if thesample is sufficiently cleaned or precleaned prior to testing.

FIG. 39 shows a sample and the analysis locations thereon as the X-Yoptics stage automatically moves the laser beam about the sample to firethe laser once at each new location as shown. In some embodiments, ahigher power (Class II or Class III) laser may be used for better signalto noise and lower detection limits. In other embodiments, an eye safe(Class I) laser is preferred.

In some examples, the calibration standard and autofocus routine can betailored based on the user's requirements. For example, if the user willlikely be interested in elemental analysis in one specific field, forexample, then wavelengths particular to that field can be used in theautofocus routine. For example, assume a user is primarily interested inthe detection of aluminum. In such an example, that customer's systemwould be equipped with an aluminum calibration standard and theautofocus routine would detect intensity data for the aluminumwavelength(s) and the autofocus routine would only focus the laser onceon the calibration standard. The optics Z-stage would then move torefocus the laser after calibration on the sample and there would be noneed to autofocus the laser on the sample at different locationsthereon. During the autofocus routine, the spectrometer output foraluminum wavelength(s) would be used to properly focus the laser beam onthe calibration standard (and then optionally on the sample at differentlocations thereon).

FIG. 40 shows shield 342 installed on end plate 340′ which is placed forinstallation on nose section portion 339′. Purge gas is delivered to theinterior of the nose section and enters the purge chamber 341, FIG. 38via one side opening 836 a, 836 h depending on the orientation of theend plate with respect to the nose portion of the analyzer. In oneexample, one side opening is blocked by the structure of the nosesection while the other side opening is not. Purge gas then flows out ofthe laser aperture and via the vent channel in the front face of the endplate to replenish the site of the plasma with fresh purge gas butwithout wasting excessive purge gas enabling a small purge gas cartridgeto be used for multiple sample analysis. FIG. 41 shows another designfor end plate 340″ with front face vent channels 850. Other designs arepossible

Thus, although specific features of the invention are shown in somedrawings and not in others, this is for convenience only as each featuremay be combined with any or all of the other features in accordance withthe invention. The words “including”, “comprising”, “having”, and “with”as used herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments.

In addition, any amendment presented during the prosecution of thepatent application for this patent is not a disclaimer of any claimelement presented in the application as filed: those skilled in the artcannot reasonably be expected to draft a claim that would literallyencompass all possible equivalents, many equivalents will beunforeseeable at the time of the amendment and are beyond a fairinterpretation of what is to be surrendered (if anything), the rationaleunderlying the amendment may bear no more than a tangential relation tomany equivalents, and/or there are many other reasons the applicantcannot be expected to describe certain insubstantial substitutes for anyclaim element amended.

Other embodiments will occur to those skilled in the art and are withinthe following claims.

What is claimed is:
 1. A handheld LIBS analyzer comprising: a lasersource for generating a laser beam; a spectrometer subsystem foranalyzing a plasma generated when the laser beam strikes a sample; and anose section including: an end plate with an aperture for the laserbeam, a purge cavity behind the aperture fluidly connected to a sourceof purge gas, a shield covering the purge cavity, and a vent forremoving purge gas from the purge cavity when the end plate is placed onthe sample.
 2. The analyzer of claim 1 in which the vent includes achannel in a front face of the end plate extending from the aperture. 3.The analyzer of claim 2 in which said channel extends on opposite sidesof the aperture to opposing edges of the end plate.
 4. The analyzer ofclaim 1 in which the rear of the end plate includes a rearwardlyextending enclosure defining the purge cavity.
 5. The analyzer of claim4 in which the enclosure includes at least one side opening forreceiving the purge gas.
 6. The analyzer of claim 4 in which theenclosure includes a top rim for seating the shield thereon.
 7. Theanalyzer of claim 1 in which the purge cavity includes a calibrationstandard therein.
 8. The analyzer of claim 7 in which the calibrationstandard is located on a rear surface of the end plate.
 9. The analyzerof claim 7 in which the end plate serves as the calibration standard.10. The analyzer of claim 8 in which there are two or more calibrationstandards located on opposite sides of the laser aperture.
 11. Ahandheld LIBS spectrometer comprising: a housing; an optics stagemovably mounted to the housing and including: a laser focusing lens, anda detection lens; one or more motors configured to advance and retractthe optics stage, move the optics stage left and right, and/or move theoptics stage up and down; a laser source in the housing oriented todirect a laser beam to the laser focusing lens; a spectrometer subsystemin the housing configured to receive electromagnetic radiation from thedetection lens and provide an output; a controller subsystem responsiveto the output of the spectrometer subsystem and configured to controlthe laser source and said one or more motors; and the housing includinga nose section with: an end plate with an aperture for the laser beam, apurge cavity behind the aperture fluidly connected to a source of purgegas, a shield covering the purge cavity, and a vent for removing purgegas from the purge cavity when the end plate is placed on the sample.12. The handheld LIBS spectrometer of claim 11 in which the shield ismade of fused silica for protecting the laser focusing lens anddetection lens of the optic stage from plasma generated by the laser.13. The handheld LIBS spectrometer of claim 11 further including a gassource fluidly coupled to the purge cavity.
 14. The handheld LIBSspectrometer of claim 13 in which the housing includes a handle and thegas source includes a gas cartridge disposed in the housing.
 15. Thehandheld LIBS spectrometer of claim 14 in which the gas cartridge ispivotably disposed in the housing handle.
 16. The handheld LIBSspectrometer of claim 15 in which the gas source cartridge is fluidlycoupled to the purge cavity via a regulator, the regulator is rotatablycoupled to the housing, and the gas source cartridge is coupled to therotating regulator.
 17. The handheld LIBS spectrometer of claim 16 inwhich the gas source is fluidly connected to the purge cavity via acontrollable valve.
 18. The handheld LIBS spectrometer of claim 17 inwhich said controller subsystem is configured to automatically controlsaid valve.
 19. The handheld LIBS spectrometer of claim 11 in which saidnose section includes a calibration standard for self-calibrating thespectrometer when the controller subsystem controls the optic stage toorient the laser focusing lens to focus laser energy on the calibrationstandard of the nose section.
 20. The handheld LIBS spectrometer ofclaim 19 in which said controller subsystem is configured to controlsaid one or more motors to move the optics stage to initiate acalibration routine.
 21. The handheld LIBS spectrometer of claim 20 inwhich said calibration routine includes computer instructions whichcontrol one or more motors to move said optics stage to a predeterminedset of coordinates, to power the laser to produce a laser beam, toprocess the output of the spectrometer subsystem, and to calibrate thespectrometer.
 22. The handheld LIBS spectrometer of claim 21 in whichsaid computer instructions which calibrate the spectrometer includeinstructions which determine wavelength and/or intensity calibrationconstants based on the output of the spectrometer subsystem.
 23. Theanalyzer of claim 21 in which the enclosure includes at least one sideopening for receiving the purge gas.
 24. The handheld LIBS spectrometerof claim 11 in which the controller subsystem is configured to controlsaid one or more motors to move said optics stage to initiate anauto-focus routine, and auto-clean routine, a moving spot cycle, and apurge cycle.
 25. The analyzer of claim 11 in which the vent includes achannel in a front face of the end plate extending from the aperture.26. The analyzer of claim 25 in which said channel extends on oppositesides of the aperture to opposing edges of the end plate.
 27. Theanalyzer of claim 11 in which the rear of the end plate includes arearwardly extending enclosure defining the purge cavity.
 28. Theanalyzer of claim 27 in which the enclosure includes a top rim forseating the shield thereon.
 29. The analyzer of claim 11 in which thepurge cavity includes a calibration standard therein.
 30. The analyzerof claim 29 in which the calibration standard is located on a rearsurface of the end plate.
 31. The analyzer of claim 29 in which thereare two or more calibration standards located on opposite sides of thelaser aperture.
 32. The analyzer of claim 29 in which the end plateserves as the calibration standard.